1. Field of the Invention
The present invention relates to a method of forming a cavity by two-step etching, and more particularly, to a method of forming a cavity having a diamond-like shape by two-step etching that can effectively reduce the dimension of a MEMS device.
2. Description of the Prior Art
Refer to FIG. 1. FIG. 1 is a schematic diagram of a conventional piezoresistive pressure sensor. As shown in FIG. 1, the conventional piezoresistive pressure sensor 10 includes a single crystalline silicon substrate 12, a diaphragm 14 disposed on the front surface of the single crystalline silicon substrate 12, a plurality of piezoresistors 16 formed in the diaphragm 14, and a cavity (back chamber) 18 formed on the back surface of the single crystalline silicon substrate 12. The cavity 18 enables the diaphragm 14 to be suspended, so that the diaphragm 14 can provide a pressure-sensing function. It is appreciated that the cavity 18 of the conventional piezoresistive pressure sensor 10 is directly formed by an anisotropic wet etching process. This anisotropic wet etching process etches the lattice plane (1,0,0) of the single crystalline silicon substrate 12, and the etching will stop at the lattice plane (1,1,1). Accordingly, the cavity 18 has an inclined sidewall.
The dimension A of the conventional piezoresistive pressure sensor 10 is decided by the size B of the bottom bonding region, the size C of the lattice projection plane, and the size D of the diaphragm 14. Specifically, A=2B+2C+D. However, the inclined sidewall of the cavity 18 is caused due to the anisotropic wet etching process, and the size C of the lattice projection plane is redundant. This redundant size C is not functional for the piezoresistive pressure sensor 10, but wastes the available areas on the single crystalline silicon substrate 12.